FIELD OF THE INVENTION
This invention realtes to methods and means of polarizing ferroelectric materials, particularly crystals, polycrystals, polymers or copolymers, such as, for example uniaxially or biaxially stretched polyvinylidene fluoride.
As soon as the aforementioned materials are polarized, and taking into account their remanent polarization, they are known to exhibit pieozoelectric and/or pyroelectric properties which make them suitable for industrial applications. The piezoelectric properties of these materials allow them to be used as emitting transducers for loudspeakers and earphones, hydrophones, echo sounding in media such as air, water, biological tissues, or as pressure transducers for microphones, chronographs for shock waves, pressure tranducers for shockwaves as radiation pressure transducers, and also as probes for echography and hydrophones.
The range of practical use of the pyroelectric properties includes temperature measurements, detection of hot points detection of intruders, and recording of infrared pictures. Within this range of practical use the ferroelectric materials are generally very thin films, the thicknesses thereof ranging from a few micrometers to one millimeter.
For the various industrial applications considered herein, both the piezoelectric and pyroelectric coefficients of the materials used must be known. Simultaneously, these coefficients must be reproducible in the manufacturing process. These coefficients are directly dependent, however, on the remanent polarization of the above-mentioned materials.
R. Hasegawa et al. (J. Polym Service A, 8, 1970), F. Micheron (Reviue technique Thomson CSF, volume 11, 3, 1979) and P.E. Bloomfield et al. (Naval Research Reviews, volume 31 No. 5, 1977) reported on methods and techniques for polarizing ferroelectric crystals, polycrystals, polymers, and copolymers. The specimen to be polarized is usually subjected to an electric field applied at room temperature or at a temperature level being in excess of or close to the Curie point. The electric field applied causes the polar axis, carrier of a permanent dipole, to be oriented in a preferred direction which is the closest one to that of the electric field. After the electric field has been removed the poled ferroelectric element exhibits a stable remanent polarization at room temperature.
Various techniques are used for the application of this electric field, examples are the method based on simple electric contacts, the corona discharge method, the plasma technique, and so forth. Because of their simplicity all these methods are undoubtedly of great advantage, but they do not allow one to get insight into the state of polarization of the material under study, that is, into the part by volume of poled material, the homogeneity of polarization on the surface and within the material, and the remanent polarization achieved. On the latter, however, depend the piezoelectric and pyroelectric coefficients which are proportional to this polarization. This is especially true for ferroelectric polymers and copolymers.
It is possible to measure the remanent polarization, but only very approximately, by measuring the total electric charge released in the case of the pyroelectric depolarization. However, the properties of the material under study set limits to such measurements which, in addition, are not accurate enough because the material heated up undergoes dielectric losses too high to be accepted.
Also known is the method reported by Sawyer and Tower (C. B. Sawyer and C. H. Tower)--Physical Review, volume 35, 269, 1930), modified by J. C. Hicks (J. C. Hicks, T.E. Jones, Ferroelectrics, volume 32, 119-126, 1981), which allows the material to be polarized as follows. A sinusoidal or triangular electric field is applied (.+-.E [MV/cm]) and the electric induction D is recorded as a function of the applied electric field E. The curve plotted in this way has the shape of a hysteresis loop D=F(E), but does not indicate the homogeneity of both the remanent and instantaneous polarization of the material because the parameter D used in the measurements takes into account the effects resulting from the ion currents or from the space charges as well as the capacitive parasitic effects emanating from the dielectric element considered. The polarization achieved in this way and the remanent polarization affect all the piezoelectric and pyroelectric properties of the material.